Calpain inhibition reduces axolemmal leakage in traumatic axonal injury

Abstract

Calcium-induced, calpain-mediated proteolysis (CMSP) has recently been implicated to the pathogenesis of diffuse (traumatic) axonal injury (TAI). Some studies suggested that subaxolemmal CMSP may contribute to axolemmal permeability (AP) alterations observed in TAI. Seeking direct evidence for this premise we investigated whether subaxolemmal CMSP may contribute to axolemmal permeability alterations (APA) and pre-injury calpain-inhibition could reduce AP in a rat model of TAI. Horseradish peroxidase (HRP, a tracer that accumulates in axons with APA) was administered one hour prior to injury into the lateral ventricle; 30 min preinjury a single tail vein bolus injection of 30 mg/kg MDL-28170 (a calpain inhibitor) or its vehicle was applied in Wistar rats exposed to impact acceleration brain injury. Histological detection of traumatically injured axonal segments accumulating HRP and statistical analysis revealed that pre-injury administration of the calpain inhibitor MDL-28170 significantly reduced the average length of HRP-labeled axonal segments. The axono-protective effect of pre-injury calpain inhibition recently demonstrated with classical immunohistochemical markers of TAI was further corroborated in this experiment; significant reduction of the length of labeled axons in the drug-treated rats implicate CMSP in the progression of altered AP in TAI.

Figure 1

Traumatically injured axons displaying HRP-accumulation indicating axonal injury (altered axolemmal permeability) in light micrographs of the corticospinal tract (A–B) from injured animals. Arrows show HRP-labeled axonal profiles. Note that the length of damaged axons appears reduced in the MDL-28170-treated (A) compared to the vehicle-treated (B) section in the CSpT and vehicle treated axons display a more vacuolized appearance. (Magnification bar indicates 20 μm.

Figure 2

Bar chart of the mean length (A) and thickness (B) of traumatically injured axons (displaying HRP-accumulation) in the corticospinal tract (CSpT) and in the medial longitudinal fasciculus (MLF) in vehicle- (vehic) and MDL-28170-treated (drug) animals subjected to impact acceleration injury. Error bars represent standard error of mean (SEM); asterisks indicate statistically significant difference of mean density values.